Gas cooking apparatus having gas-radiation burners, glass-ceramic cook plates and control units for the gas supply are disclosed, for example, in German patent publications 3,934,562 and 2,621,801 and U.S. Pat. 3,468,298; 4,083,355 and 4,201,184.
In gas-radiation burners, the gas is combusted at the surface of a burner plate comprising a porous ceramic. In gas cooking apparatus, one or more of such gas-radiation burners are arranged at a spacing below a common glass-ceramic plate known per se. A cooking position is defined by each burner at the upper side of the glass-ceramic plate. Each individual gas-radiation burner is provided with an ignition device and protected by an ignition safety device against an outflow of uncombusted gas mixture. A burner plate of this kind is disclosed, for example, in European patent publication 0 187 508.
The temperature of the radiating burner plate lies in the range between approximately 900.degree. C. and 950.degree. C. depending upon the temperature properties of the material. The maximum gas quantity which can be supplied to the burner is limited by constructive measures via control devices so that a maximum operating temperature cannot be exceeded, for example, to protect the material of the burner plate or the cooking surface and also to avoid unwanted energy losses.
The permissible maximum temperature of glass-ceramic cook plates lies usually between approximately 700.degree. C. and 750.degree. C.
For an adjusted high power, temperatures of 900.degree. C. and more can occur within a short time in the glass-ceramic plate because of pots having unsuitable bases (especially uneven bases) or when cooking positions are tightly occupied. For this reason, a temperature limiter is provided to protect the glass-ceramic plate. With the temperature limiter, such overtemperatures can be reliably prevented. Such temperature limiters are described in detail, for example, in German patent publication 2,621,801 and in U.S. Pat. No. 4,201,184.
For heating a cooking area, a control (closed loop or open loop) of the power of the burner must be provided in addition to a limitation on temperature. Two principles are known to control the power: in the first principle, the burner is continuously operated and the supplied gas quantity is decreased or increased in correspondence to the required power. According to the second principle, the burner is driven in a clocked manner, that is, the burner is always driven with the maximum gas quantity and the required power is obtained from the ratio of the switch-on time to the switch-off time (pulse duty factor).
German patent publication 3,315,745 discloses a cooking field including: gas-heated burners; a continuous cook plate of glass ceramic or a comparable material with at least two clearly separated cooking zones, which have separate burner locations assigned thereto; a warm-holding zone; exhaust-gas channels for conducting the combustion gases away; and ancillary units. The burner locations include burner chambers, gas-mixing chambers, gas-mixing devices and control devices. The burner locations, the warm-holding zone and the exhaust-gas channels at the regions, which do not serve the heat transfer to the cooking field, as well as all components of the burner locations are surrounded by a component comprising low mass and heat insulating material which is in common with these parts.
German patent publication 3,315,745 discloses that the necessary overpressure in the mixture chamber is reached by only a cross-sectional narrowing within a combustion-mixture supply channel. For this gas-mixing device, a conical bore together with the gas nozzle is effective as an injector through which the combustion air is drawn in by suction.
The control unit for the gas supply disclosed in German patent publication 3,315,745 has no valve blocks with exchangeable nozzles and can therefore only be adapted to different gas types with great difficulty. Furthermore, and notwithstanding the reduction in cross section within the combustion mixture supply channel, the disadvantage is present that insufficient combustion air is supplied when the gas is not injected into the combustion mixture supply channel with an adequately high prepressure. A unit with which additional air is supplied is however not provided.
German patent publication 3,844,081 discloses a cooking apparatus with at least one cooking field comprising a burner pot seated in a burner plate, a nozzle plate and a ceramic plate mounted at a spacing above the nozzle plate. An intermediate base wall is mounted below the burner pot and a compartment for open-loop control and/or closed-loop control and/or monitoring devices is provided below the intermediate base wall. An exhaust-gas discharge is provided wherein, on the rearward side of the cooking apparatus, an essentially vertical shaft is arranged which can be connected to an exhaust channel or is configured as such. A blower is mounted in the compartment for the open-loop control and/or closed-loop control and/or monitoring apparatus. The blower inducts primarily cooling air from the front end through this compartment and this cooling air is forced into the shaft. The shaft is configured as a flow channel so that the primary cooling air draws in secondary cooling air above the spacer chamber between the burner plate and the intermediate base wall and conducts away exhaust gas from the compartment between the burner plate and the ceramic plate.
German patent publication 3,844,081 provides a cooking apparatus of the described configuration with reference to controlling the heat loss and therefore to improve the same. Here, a blower of relatively high complexity is utilized simply to remove the exhaust gas. Also, a control device adapted to different types of gas is not provided.
U.S. Pat. No. 4,020,821 discloses positioning a blower in a gas cooking apparatus equipped with a continuous plate, for example, made of glass ceramic. This blower brings in additional air for a good combustion of the gas. However, here, no gas-radiation burners having burner plates are utilized; instead, burners with an open flame are used which require different control devices.
British patent publication 2,230,595 discloses a gas hob having a glass-ceramic plate and at least one gas-radiation burner mounted closely under the plate. Each burner unit has a burner chamber with a large number of individual chambers. A burner plate made of massive ceramic is mounted above the individual chambers. The burner plate is adaptively perforated to arrange the chambers. Furthermore, the gas hob includes a gas supply and a ventilator unit to always make combustion air available when the burner unit operates.
U.S. Pat. No. 4,020,821 and British patent publication 2,230,595 both show the use of ventilators or blowers in gas cooking apparatus having continuous glass-ceramic cooking plates; however, the overall arrangement of the cooking apparatus including the control units for the gas supply are very complicated and complex and therefore correspondingly susceptible to malfunction in practical use as well as being expensive in manufacture. Furthermore, these units are inflexible with respect to a change of gas type.
None of the publications of the state of the art provide any suggestion as to pressure control units for the adjustment of a gas supply free of pressure fluctuations and for controlling to a minimum operating pressure which is necessary for the operation of a burner.
German patent publication 4,225,789 discloses that gas apparatus can be adjusted with the aid of a gas pressure controller to a specific pressure or throughput. According to this publication, the gas pressure, which is pregiven by the gas supply company, is controlled to a specific but variable desired value so that a gas prepressure results, which is reduced via a plurality of reference nozzles to the apparatus connection pressure. The reference nozzles can each be selected via a control. In this way, an adjustable gas throughput is achieved even for different gas types and input gas pressures which change. The method shown in German patent publication 4,225,789 is very complex and cannot be applied in order to be able to drive a gas-radiation burner as in the present invention for a very low gas prepressure. In such a case, the combustion air must be introduced with the support of a blower. Only in this way does the gas burn reliably and cleanly.
German patent publication 4,326,945 discloses a gas cooking apparatus with at least one gas-radiation burner having a burner chamber and a burner plate made especially of fiber material. The gas-radiation burner is arranged below a continuous cooking plate made of a material such as glass ceramic, glass, ceramic or a similar material which is permeable for thermal radiation. The gas cooking apparatus includes control units for the gas supply as well as conventional ignition, safety and temperature monitoring devices. The apparatus also includes exhaust-gas channels for conducting away the combustion gases as well as a unit, such as a blower, with which additional air is brought to the burner plate. The device with which the additional air is inducted from the outside and brought to the burner plate and the control unit for the gas supply are both located in a compartment which is partitioned off and in which an overpressure is maintained. The compartment communicates with the combustion chamber via lines which are gas tight in the region outside of the partitioned chamber (especially via tubes with the combustion chamber) and with the ambient. The control unit for the gas supply includes: a valve block which, in turn, includes especially a magnetic control; and, nozzles positioned in the block, which can be exchanged and have opening diameters of 0.5 mm to 2.0 mm (especially 1 mm) adaptable to various gas types.
It is problematic with all gas-radiation burners not equipped with their own pressure control units for maintaining a pressure-fluctuation free gas supply that such burners react very sensitively to gas pressure fluctuations. If the pressure drops off greatly below the nominal pressure, then this leads to a very air-rich gas/air mixture and therefore to a poor efficiency accompanied by reduced cooking capacity. If the pressure falls very far below nominal pressure, then this leads to a complete malfunction of the apparatus because the burner no longer ignites because of too much excess air. The pressure can fall below the nominal pressure in many countries and regions especially during winter. Italy is exemplary here where, in lieu of a normal natural gas pressure of 17 to 25 mbar, only 8 mbar can be available in winter. Without a pressure control unit, a reliable operation of the gas-radiation burners is no longer ensured. The pressure control unit supplies the control unit for the gas supply with a pressure-fluctuation free and constant operating gas pressure.